To write a novel, you need to decide who is seeing the action, who is narrating the story and what is the point of view (POV).
Each point of view will dramatically change the novel.
Try this at home. When something happens, ask each member of your family to describe what happened. You can be sure that every person will see things differently and will interpret what happened with their own particular background and experiences.
The hardest thing to do when writing from a single POV is to see things through the eyes of one single character. It is easy to introduce thought that do not belong to the POV.
WRONG: Jill watched her group of friends. They were talking about baseball, again. (A friend’s POV)
How does your POV know what they are talking about? If Jill watches the group from a distance, she cannot possibly hear their conversation. This has to be a POV switch where we look at the scene with Jill then switch to the group and hear what they are talking about through Jill's friend's ears.
The best way to rewrite this sentence is to say:
RIGHT: Jill watched her group of friends. They were swinging their arms and imitating someone throwing a ball. They were probably talking about baseball, again. (Jill POV)
WRONG: Riley was the happiest person in the bunch. Of course, her baseball player boyfriend had asked her to marry him. (That’s Riley’s POV)
RIGHT: Riley seemed happy. She smiled non-stop and she looked at her engagement ring every second. (This is Jill’s POV)
The story can have different POVs, but it is better to reserve at least 70% of the narration to one unique POV because the POV switches give the impression that the story is disjointed and the reader gets confused. And the reader can only get close to one narrator.
The POV should go to the character who is the most involved emotionally in the story; the one who will ultimately be the most changed by what is happening in the story.
The reader will be influenced by the POV’s feelings, thoughts and actions throughout the book. Never forget, even for an instant, who is your POV.
WRONG: Jill followed the team inside the gym. Groups of players were already in the showers and others lingered with their family members, discussing the outcome of the match. Riley had already found a corner to kiss her future husband.
RIGHT: Jill followed the team inside the gym. Players stepped in a bunch toward the cloakroom. Others had split up into groups with their friends and families. Riley pulled her future husband in a dark corner. Jill remembered how Riley had boasted about her engagement. It made Jill’s heart ache. Jill sighed. Life went on.
To keep to one single POV allows the reader to sympathize with one character and feel close to her or even identify with her. On the contrary, switching POVs causes the reader to distance himself from the main character and does not allow the reader to cheer for one person.